Stop bands in multifilar helices



Nov. 22, 1960 D. G. DOW

STOP BANDS m MULTIFILAR HELICES Filed July 25, 1959 INVENTOR.

DAN/EL 6. Dow

ATTORNEY United States Patent S1 01? B NDS IN MULTLFILAR BEL-ICES Daniel G. Dow, Altadena, Califl, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed July 23, 1959, SeraNo. 829,168,-

Claims. (.Cl..315e-3..6

This invention relates to stop bands in multifilar helices and more particularly to a slow-wave structure which forms the microwave circuit portion of a traveling-wave amplifier tube, when used with an appropriate high voltage electron beam.

An object of the invention is to provide a travelingwave amplifier tube, which has a large 3-db bandwidth and which is very nearly free from self-oscillation caused by beam interaction with a backward space-harmonic of the structure having a velocity very nearly synchronous with the electron beam. By the aforementioned description of bandwidth is meant to those skilled in the art that the ratio of the frequencies at the 3-db down points on the response curve ranges from 1.5 :1 to 2:1

or more.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Fig. l is a more or less schematic view of a travelingwave tube constituting a preferred embodiment of the invention;

Fig. 2 is an enlarged axonometric view of a portion of the bifilar helix removed from the tube of Fig. 1;

Fig. 3 is a cross-section taken along the line 3-3 of Fig. 1;

Fig. 4 is a cross-section taken along the line 44 of Fig. 1;

Reference is now made to the drawing. The slowwave structure of this invention comprises a multifilar helix illustrated in a preferred embodiment as being a bifilar helix. The bifilar helix is shown as being made of two separate helices 2 and 4 of uniform conducting tapes of width W and thickness T. Each has the form of a helix of constant pitch p (the axial distance in which the conductor makes one complete rotation about the axis), constant pitch angle \l/ (the angle between a line tangent to an edge of one of the tapes and a plane perpendicular to the cylindrical axis of the helix), and constant mean radius a. The helices are spaced equally from each other a distance d.

The helices are connected across the center of the structure every distance d by means of conducting bars (straps), every other one of which 8 has a small gap 10 in the center. The cross-section of these bars is shown to be circular, but can be square, rectangular or any other convenient cross-section. The spacing of the straps is dictated by the desired location of the stop band, as explained hereinafter, and will generally be nearly that shown in the drawing (half the pitch distance p).

The exact cross-section of the helix conductors (shown to be tapes) and the straps (shown to be wires) is of secondary importance to their configuration.

The multifilar helix of the invention is shown installed in a conventional traveling wave tube generally indicated at 12 which includes an electron gun 14, a collector 16,

Patented Nov. 22, 1960 input terminals 18,. and output terminals 20. The helix can be supported by; dielectric rods running through its interior (omitted for the sake of clarity) or, if the tube axis is: vertical, the helix: can be supported, bythe input and, output terminals.

Operation- To understand the operation of the present invention, it is convenient to consider first an unstrapped bifilar helix as a slow-wave structure. The unstrapped bifilar helix has an infinity of possible modes. of propagation, two of which areimportant here. The so-called even mode is characterized by fields atthe twohelices, being in phase with each other. It has a forward-traveling space harmonic which is useful for wide band forwardwave amplification, and a moderately strong -2 backward space harmonic which may cause oscillations at extremely high currents. An odd order or push-pull mode also exists, for which the fields at the two helices are out of phase at any cross-section. This mode has a very strong backward space harmonic that will generally cause oscillations under the operating conditions necessary for forward amplification.

The present invention introduces a strong stop band in the odd mode in such a way as to prevent backwardwave oscillation, while at the same time conserving the characteristics of the even mode. When the symmetry properties of the structure are considered it is seen that along the axis of the helix there will be a magnetic field maximum in the odd mode and a magnetic field zero in the even mode. It has been shown that placing straps diametrically across the bifilar helix will have only a very small eifect on the forward-wave characteristics. Within these straps there will be a maximum of current ae the center. Thus, the gaps of the present invention represent a very large perturbation to this mode with a periodic spacing 2d. Since there is no current at the center in the even mode, the gaps will not affect it, and thus the structure of the present invention achieves a different basic periodicity for the two modes. In particular, the fundamental period of the odd mode is such as to introduce a stop band in just such a manner as to prevent backward-wave oscillation in the odd mode. Although the structure of the present invention involves several space harmonics in the odd mode, their relation to the beam velocity is such that they cannot give backward-wave oscillations and since these oscillations are impossible, the tube can be operated as a forward-wave amplifier at high power levels.

Although the illustrated embodiment shows a bifilar helix, nevertheless, as previously suggested, the invention comprehends the use of multifilar helices using any number of helices. Each helix would be identical to the others and the helices would be spaced evenly along their common axis. If the number of helices is n the straps will be composed of 11 bars joined at the center. To introduce the stop band, the bars of every mth set will not be joined at the center, but will be left with a small gap. Here m is an integer which will usually be the same as n, but need not be in general.

The helices are shown in the preferred embodiment as tapes, while the straps are shown as rods. As a practical matter the exact cross-section of either is not important, but may be of any cross-section that is convenient.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. A slow-wave structure for use in traveling-wave tubes comprising a multifilar helix including a plurality of n separate helices, wherein n is any integer, each separate helix having the same constant pitch, the same constant pitch angle and the same constant mean radius, said helices being spaced equally from each other along a common axis; and a plurality of conducting straps, spaced periodically from each other along the direction of the axis of the helix, electrically connecting the helices together across a diameter of the helix, each strap being composed of n bars joined at the axis of the helices with the exception that the bars of every mth set are provided with a gap at the axis of the helices; m being an integer.

2. The slow-wave structure of claim 1 wherein the quantity m equals the quantity n.

3. The slow-wave structure of claim 1 wherein the 15 multifilar helix is a bifilar helix.

4. The slow-wave structure of claim 2 wherein the straps are spaced from each other a distance equal to half the pitch distance of each separate helix.

5. In a traveling-wave tube an electron gun, a col- 5 lector for electrons, and a slow-wave structure; said slowwave structure being the structure of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,806,975 Johnson Sept. 17, 1957 2,809,321 Johnson et al. Oct. 8, 1957 2,889,487 Birdsall et al. June 2, 1959 2,898,507 Clarke Aug. 4, 1959 

